Allow group owners to delete users of Enterprise Users

Proposal

Administrator should have the ability to delete enterprise users in their namespace either

1. manually through the UI
2. programatically through the User API
3. automatically through an option in SCIM during the de-provision process.

A clear warning message would be required to indicate that this will unlink any user contributions and replaced with a Ghost user. And that this is a destructive action that cannot be reversed.

I imagine most user will not use this destructive method though there are some use cases where users created erroneously during an automated import from SCIM results in a bulk of users being created that do not have any need to remain on GitLab.com. In such cases it would be beneficial for owner to be able to essentially roll these users back via an acceptable means with no risk of data loss.

This is especially true if occupied usernames and emails are to be used differently for service accounts or a different company (federated).